This invention relates to a device for drawing the knitwork in a circular knitting machine of the double cylinder type. More particularly, the invention concerns a device of the fluid-operated type, having a piston with a rod which is movable axially within one of the needle cylinders and includes a knitwork engagement means for keeping the knitwork stretched or under tension while being formed, said means rotating together with the needle cylinders and knitwork.
A device of the same general type is described in the Italian Pat. No. 718,780. According to that Patent, the piston is movable in a cylindrical chamber arranged stationary above the upper needle cylinder of the machine, and the rod passes through the upper cylinder and carries, mounted pivotally to its bottom end, a cup-like element which is provided with downward sloping radial pointed fingers for engagement with the knitwork being knitted.
During the knitwork formation, pressurized fluid is discharged from the cylindrical chamber, and the piston allowed to move downwards under its own weight, thus putting under tension the knitwork being knitted, wherethrough the ends of the cup element fingers are caused to penetrate. On completion of the knit article, the piston is raised by admitting fluid under pressure into the cylindrical chamber to restore the initial conditions.
U.S. Pat. No. 3,750,426 discloses a device of that same general type, with the exception that the piston now carries pivotally several rods, whereto the cup element is rigid. The rods are rotated by the upper needle cylinder itself, such as to relieve the knitwork or fabric of any rotary drag of the cup element.
However, a device to the same design as disclosed in the cited patents requires the provision for a cylindrical chamber of considerable height above the upper needle cylinder, that height dimension being substantially equal to the piston stroke length, i.e. to the length of the knitwork article to be stretched. This results in a considerable increase of the machine overall height.
According to another solution, set forth in the U.S. Pat. No. 3,797,280, it is possible to reduce the height of the cylindrical chamber and the stroke length of the piston by arranging a second piston in a second chamber extending coaxially with the first chamber, the pistons being actuated at different times, thereby while the main or master piston returns upwards, the tensioning action is provided by the second piston, and viceversa.
This approach, additionally to affording a reduction in the machine height, also ensures continuous stretching, but involves a comparatively complex structure for the arrangement of two cylindrical chambers and two pistons, as well as two sets of rods, to be provided each with knitwork engagement means of their own.